Light-emitting diodes (LEDs) have been extensively used for general lighting because of their desirable features such as long life, high energy efficiency, and design flexibility. Most households are powered by alternating current (AC) voltage mains, and thus stacks of LEDs may be directly connected to an AC voltage outlet without any direct current (DC) conversion. To fit AC excitation, a pair of LEDs (or two groups of back-to-back LEDs) may be connected in an anti-parallel arrangement, in which the two LEDs in the pair are connected in parallel, but with their orientations or polarities reversed. LEDs may be paired this way to protect each other from reverse voltage. A series string or stack of such pairs can be connected to an AC voltage source, and the LEDs in each pair take turns emitting light, on alternate half-cycles of the voltage source.
As each LED has a threshold voltage under which no current may flow through the LED, in an AC cycle around the time the voltage source crosses a zero point, there may be two periods of off-time in which no current flows through a pair of anti-parallel LEDs. To maximize light output and increase energy efficiency, it is therefore desirable to minimize the off-time of LEDs during AC operation.